Machine tools, including drills, lathes, milling machines, grinders and other finishing machines, and more complex devices such as screw machines, are all characterized by a common objective: the manufacture of large numbers of identical finished parts under conditions of extreme accuracy and maximum economy. As such, interest in and development of machine tools has paralleled the advance of the industrial revolution.
Traditionally, machine tools were operated by machinists who were among the most highly skilled and the most highly paid of all workers. More recently, however, machine tools have been adapted to a procedure known as computer numeric control, or CNC, whereby the operation of machine tools is regulated by computers or other programmable controllers. In accordance with the CNC technique, the dimensions, surface finishes, and other characteristics of the part to be manufactured are supplied in the form of sequential operating instructions which are utilized by the CNC device to regulate the operation of the machine tool. This allows the completion of finished parts with more uniformity and more rapidity than has ever been possible heretofore.
The adaptation of single spindle lathes, milling machines, and similar devices to CNC techniques has largely been successful. However, in the case of multi-spindle machine tools, previous attempts at automation have largely comprised adapting the cams, gears, and other components comprising such machines to servo control. Perhaps because the approach has been one of adapting old designs to new techniques, the effort to date at automating the operation of multi-spindle lathes by means of CNC operation has largely been unsuccessful.
The present invention comprises a multi-spindle lathe which is entirely adapted for CNC operation. In accordance with the broader aspects of the invention, a plurality of spindles are positioned at spaced points about a central axis. Each spindle has a collet which receives a length of stock and rotates the stock about a spindle axis. An indexing mechanism is provided for selectively positioning the spindles at work stations located at equally spaced points about the central axis.
Each work station comprises an internal tool slide adapted to receive a cutting tool and to advance the cutting tool toward and away from the rotating stock under the action of a servo mechanism. An external tool slide is also provided for each work station and is adapted to advance a cutting tool both toward and away from and parallel to the axis of rotation of the stock. At each work station the stock is turned rather than formed, meaning that the cutting tools of the individual work stations may be utilized to perform a variety of quite distinct machining operations.
The multi-spindle CNC lathe of the present invention is readily adapted for use in conjunction with both the Just In Time (JIT) and the Statistical Process Control (SPC) manufacturing philosophies. In accordance with JIT, only the exact number of piece parts necessary to complete a particular assembly operation are ordered at any one time. This eliminates the investment in inventory which is necessary when large numbers of piece parts are ordered simultaneously, and also eliminates the possibility that previously ordered parts will become obsolete due to a change in design. The machine tool of the present invention is adapted to JIT because the economic batch is smaller. This is because machine tools incorporating the invention do not require the changing of the cutting tools utilized at the various work stations in order to change the nature of the piece parts being manufactured, and because set up time is reduced dramatically.
In accordance with SPC, completed piece parts are compared with a predetermined standard with a view towards maintaining the dimensions of each part at the center of the tolerance range. If the dimensions of the parts being manufactured begin to vary from the center of the tolerance range, due to cutting tool wear or otherwise, adjustments in the manufacturing process are immediately instituted in order to maintain tolerances. SPC is easily practiced in the machine tool of the present invention since all of the cutting tools are positioned by servo mechanisms which are in turn under computer numeric control.